Optical scanner and image forming apparatus

CROSS-REFERENCE TO RELATED APPLICATIONS

0001The present document incorporates by reference the entire contents of Japanese priority document, 2003-328941 filed in Japan on Sep. 19, 2003.

BACKGROUND OF THE INVENTION

00021) Field of the Invention

0003The present invention relates to an optical scanner and an image forming apparatus, and particularly to an optical scanner and an image forming apparatus that are utilized for optical printer, digital copying machine, plain-paper facsimile, and the like.

00042) Description of the Related Art

0005In an image forming apparatus utilizing an electrophotographic process used in optical printer, digital copying machine, plain-paper facsimile, and the like, the colorization and faster printing have become common, and a tandem type image forming apparatus using a plurality of photoconductors is becoming widespread. In the tandem type image forming apparatus, a larger number of light sources are utilized and consequently the number of components for the apparatus is increased, and this results in a cost increase for the apparatus. In the image forming apparatus, failure of light sources can be cited as the main cause for malfunctions of units that perform optical scanning, and as the number of light sources is increased, a probability of malfunctions becomes higher.

0006The present inventor has proposed a tandem type image forming apparatus with a reduced number of light sources as disclosed in the Japanese Patent Application Laid-open No. 2002-23085.

0007In the image forming apparatus described in the patent application, optical scanning paths from one light source toward two photoconductors are formed, the optical scanning paths toward the two photoconductors are sequentially selected with a time lag, and the light beams from one light source are intensity-modulated with image signals corresponding to an optical scanning position optically scanned by the selected optical scanning path, thereby the two photoconductors are optically scanned with light beams from one light source.

0008Accordingly, since the two photoconductors can be optically scanned with one light source, only two light sources are required even in the tandem type image forming apparatus using four photoconductors.

0009However, in an embodiment specifically described in the patent application, there is a limitation to speeding-up of image formation because single optical scanning on two photoconductors is performed with the light beams from one light source by one rotation of a optical deflector.

SUMMARY OF THE INVENTION

0010It is an object of the present invention to solve at least the problems in the conventional technology.

0011An optical scanner according to an aspect of the present invention is configured to optically scan n optical scanning positions with a light beam from a light source by forming optical scanning paths from the light source to the n optical scanning positions, where n≧2, sequentially selecting the optical scanning paths at different timings, and performing intensity modulation of the light beam from the light source with image signals corresponding to the optical scanning positions optically scanned via the optical scanning path selected, the optical scanner comprising: m light sources configured to emit p light beams, where 1≦m<n and p≧1; a light beam splitting unit configured to split each of the light beams from each of the light sources into q light beams in a sub-scanning direction and to form each of the light beams into a predetermined beam form, where 2≦q≦n; a polygon mirror type optical deflector including a plurality of deflection-reflection planes around a rotation axis and configured to deflect a number of light beams equal to (q·m·p) from the m light sources which are incident on the polygon mirror type optical deflector; n scanning imaging optical systems configured to guide n sets of light beams deflected by the polygon mirror type optical deflector to corresponding optical scanning positions to form light spots; and an optical path selecting unit configured to select an optical scanning path for each of the n sets of light beams, wherein the polygon mirror type optical deflector is at least a part of the optical path selecting unit configured to select an optical scanning path for each of the n sets of light beams, wherein the polygon mirror type optical deflector is at least a part of the optical path selecting unit.

0012A tandem type image forming apparatus according to another aspect of the present invention, in which optical scanning is performed on a plurality of photoconductors with an optical scanner according to the above aspect so as to individually form electrostatic latent images, the electrostatic latent images are visualized to form toner images, and the toner images are transferred onto a same sheet recording medium to synthetically perform image formation.

0013The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0014<figref idref="DRAWINGS">FIG. 1</figref> is a view for explaining one embodiment of an optical scanner;

0015<figref idref="DRAWINGS">FIG. 2</figref> is a view for explaining an operation of a half mirror prism in <figref idref="DRAWINGS">FIG. 1</figref>;

0016<figref idref="DRAWINGS">FIG. 3A</figref> is a view for explaining a function of a polygon mirror type optical deflector in <figref idref="DRAWINGS">FIG. 1</figref>;

0017<figref idref="DRAWINGS">FIG. 3B</figref> is a view for explaining the function of the polygon mirror type optical deflector in <figref idref="DRAWINGS">FIG. 1</figref>;

0018<figref idref="DRAWINGS">FIG. 4</figref> is a view for explaining an intensity modulation of a light source when black and magenta images are written in an embodiment of <figref idref="DRAWINGS">FIG. 1</figref>;

0019<figref idref="DRAWINGS">FIG. 5</figref> is a view for explaining an example in which light emission intensity values of the light source are different from each other when the black and the magenta images are written by the intensity modulation of <figref idref="DRAWINGS">FIG. 4</figref>;

0020<figref idref="DRAWINGS">FIG. 6</figref> is a view for explaining an example of a light beam split;

0021<figref idref="DRAWINGS">FIG. 7</figref> is a view for explaining another example of a light beam split;

0022<figref idref="DRAWINGS">FIG. 8A</figref> is a view for explaining light incidence of a polygon mirror type optical deflector;

0023<figref idref="DRAWINGS">FIG. 8B</figref> is a view for explaining light incidence of the polygon mirror type optical deflector;

0024<figref idref="DRAWINGS">FIG. 8C</figref> is a view for explaining light incidence of the polygon mirror type optical deflector;

0025<figref idref="DRAWINGS">FIG. 9</figref> a view for explaining one embodiment of the optical scanner; and

0026<figref idref="DRAWINGS">FIG. 10</figref> is a view for explaining one embodiment of an image forming apparatus using the optical scanner of <figref idref="DRAWINGS">FIG. 9</figref>.

DETAILED DESCRIPTION

0027Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings.

0028<figref idref="DRAWINGS">FIG. 1</figref> is a view for explaining an embodiment of the optical scanner according to the invention. In <figref idref="DRAWINGS">FIG. 1</figref>, reference signs <b>1</b> and <b>1</b>′ represent semiconductor lasers. The semiconductor lasers <b>1</b> and <b>1</b>′ are two light-emission sources constituting one light source, and each of the semiconductor lasers emits one light beam. The semiconductor lasers <b>1</b> and <b>1</b>′ are held in a predetermined positional relationship by a holder <b>2</b>.

0029Each of the light beams emitted from the semiconductor lasers <b>1</b> and <b>1</b>′ is converted into a luminous flux form (parallel luminous flux or luminous flux with weak divergence or with weak convergence) suitable for subsequent optical systems by coupling lenses <b>3</b> and <b>3</b>′. In this example, the light beams coupled by the coupling lenses <b>3</b> and <b>3</b>′ are parallel luminous flux.

0030Each light beam outgoing from the coupling lens to become a desired luminous flux form is beam-shaped by passing through an opening of an aperture <b>12</b> that restricts a light beam width, each light beam then enters a half mirror prism <b>4</b>, and is split into two light beams in a sub-scanning direction by an operation of the half mirror prism.

0031<figref idref="DRAWINGS">FIG. 2</figref> depicts the state that the light beam being split into two light beams. For simplicity of the drawing, only a light beam L<b>1</b> emitted from the semiconductor laser <b>1</b> is shown in <figref idref="DRAWINGS">FIG. 2</figref>. The vertical direction of <figref idref="DRAWINGS">FIG. 2</figref> is the sub-scanning direction, and the half mirror prism <b>4</b> has a semitransparent mirror <b>4</b><i>a </i>and a reflecting plane <b>4</b><i>b </i>that are parallel to each other in the sub-scanning direction. The light beam L<b>1</b> enters the half mirror prism <b>4</b>, and then enters the semitransparent mirror <b>4</b><i>a</i>. A part of the incident light beam is transmitted straight through the semitransparent mirror <b>4</b><i>a </i>to become a light beam L<b>11</b>. The remaining part of the light beam is reflected from the semitransparent mirror <b>4</b><i>a </i>to enter the reflecting plane <b>4</b><i>b</i>, and is totally reflected from the reflecting plane <b>4</b><i>b </i>to become a light beam L<b>12</b>.

0032In this example, since the semitransparent mirror <b>4</b><i>a </i>and the reflecting plane <b>4</b><i>b </i>are also parallel to each other, the light beams L<b>11</b> and L<b>12</b> outgoing from the half mirror prism <b>4</b> are also parallel to each other. Thus, the light beam from the semiconductor laser <b>1</b> is split into the two light beams L<b>11</b> and L<b>12</b> in the sub-scanning direction. The light beam from the semiconductor <b>1</b>′ is split into two in a similar manner.

0033Accordingly, the two light beams are emitted from one light source (m=1) and each of the two light beams is further split in the sub-scanning direction into two light beams (q=2). As a result, the four light beams are obtained.

0034Returning to <figref idref="DRAWINGS">FIG. 1</figref>, the four light beams are incident to cylindrical lenses <b>5</b><i>a </i>and <b>5</b><i>b</i>. The four light beams are focused into the sub-scanning direction by the operation of the cylindrical lenses <b>5</b><i>a </i>and <b>5</b><i>b </i>and imaged as a long line image in a main scanning direction near a deflection reflection plane of a polygon mirror type optical deflector <b>7</b>.

0035As shown in <figref idref="DRAWINGS">FIG. 1</figref>, among the light beams emitted from the semiconductor lasers <b>1</b> and <b>1</b>′ and split by the half mirror prism <b>2</b>, the light beam (light beam L<b>11</b> shown in <figref idref="DRAWINGS">FIG. 2</figref>) transmitted straight through the semitransparent mirror <b>4</b><i>a </i>of the half mirror prism <b>4</b> enters the cylindrical lens <b>5</b><i>a</i>, and the light beam (light beam L<b>12</b> shown in <figref idref="DRAWINGS">FIG. 2</figref>), which is reflected from the semitransparent mirror <b>4</b><i>a </i>and further reflected from the reflecting plane <b>4</b><i>b</i>, enters the cylindrical lens <b>5</b><i>b. </i>

0036In <figref idref="DRAWINGS">FIG. 1</figref>, a reference sign <b>6</b> represents sound-proof glass that is provided in a window of a sound-proof housing of the polygon mirror type optical deflector <b>7</b>. The four light beams from the light source side enter the polygon mirror type optical deflector <b>7</b> through the sound-proof glass <b>6</b>, and deflected light beams are emitted through the sound-proof glass <b>6</b> toward a scanning imaging optical system side.

0037In the polygon mirror type optical deflector <b>7</b>, an upper polygon mirror <b>7</b><i>a </i>and a lower polygon mirror <b>7</b><i>b </i>are integrally formed in two stages in the direction of a rotation axis. The polygon mirror type optical deflector <b>7</b> is rotated around the rotation axis by a drive motor (not shown).

0038In this example, the upper polygon mirror <b>7</b><i>a </i>and the lower polygon mirror <b>7</b><i>b </i>have an identical shape and both include four deflection reflection planes, and the deflection reflection plane of the lower polygon mirror <b>7</b><i>b </i>is shifted in a rotation direction by a predetermined angle θ (=45°) relative to the deflection reflection plane of the upper polygon mirror <b>7</b><i>a</i>. The upper polygon mirror <b>7</b><i>a </i>and the lower polygon mirror <b>7</b><i>b </i>may be integrally formed.

0039In <figref idref="DRAWINGS">FIG. 1</figref>, reference signs <b>8</b><i>a </i>and <b>8</b><i>b </i>represent a first scanning lens, reference signs <b>10</b><i>a </i>and <b>10</b><i>b </i>represent a second scanning lens, reference signs <b>9</b><i>a </i>and <b>9</b><i>b </i>represent an optical path folding mirror, and reference signs <b>11</b><i>a </i>and <b>11</b><i>b </i>represent a photoconductor.

0040The first scanning lens <b>8</b><i>a</i>, the second scanning lens <b>10</b><i>a</i>, and the optical path folding mirror <b>9</b><i>a </i>constitute a set of scanning imaging optical system that guides the two light beams deflected by the upper polygon mirror <b>7</b><i>a </i>of the polygon mirror type optical deflector <b>7</b> (two light beams emitted from the semiconductor lasers <b>1</b> and <b>1</b>′ and transmitted through the semitransparent mirror <b>4</b><i>a </i>of the half mirror prism <b>4</b>) onto the photoconductor <b>11</b><i>a</i>, at a corresponding optical scanning position, to form two light spots that are separated from each other in the sub-scanning direction.

0041The first scanning lens <b>8</b><i>b</i>, the second scanning lens <b>10</b><i>b</i>, and the optical path folding mirror <b>9</b><i>b </i>constitute a set of scanning imaging optical system that guides the two light beams deflected by the lower polygon mirror <b>7</b><i>b </i>of the polygon mirror type optical deflector <b>7</b> (two light beams emitted from the semiconductor lasers <b>1</b> and <b>1</b>′ and transmitted through the semitransparent mirror <b>4</b><i>a </i>of the half mirror prism <b>4</b>) onto the photoconductor <b>11</b><i>b</i>, at a corresponding optical scanning position, to form two light spots that are separated from each other in the sub-scanning direction.

0042For the light beams emitted from the semiconductor laser <b>1</b> and <b>1</b>′, an optical arrangement is set as principal rays to intersect near a position of the deflection reflection plane viewed from the rotation axis direction of the polygon mirror type optical deflector <b>7</b>. Accordingly, in each pair of two luminous fluxes enter the deflection reflection plane, the light beams both have an open angle (an angle formed by projections to the plane orthogonal to the rotation axis of the two light beams when the light source side is viewed from the deflection reflection plane side).

0043The two light spots formed on each of the photoconductors <b>11</b><i>a </i>and <b>11</b><i>b </i>are also separated from each other in the main scanning direction by the open angle. Therefore, the two light beams optically scanning on each photoconductor can be individually detected to synchronize a start of the optical scanning in each light beam.

0044Accordingly, the multi-beam scanning is performed on the photoconductor <b>11</b><i>a </i>by the two light beam deflected by the upper polygon mirror <b>7</b><i>a </i>of the polygon mirror type optical deflector <b>7</b>, and the multi-beam scanning is performed on the photoconductor <b>11</b><i>b </i>by the two light beam deflected by the lower polygon mirror <b>7</b><i>b </i>of the polygon mirror type optical deflector <b>7</b>.

0045Since the deflection reflection planes of the upper polygon mirror <b>7</b><i>a </i>and of the lower polygon mirror <b>7</b><i>b </i>of the polygon mirror type optical deflector <b>7</b> are shifted by 45° from each other, the light beams deflected by the lower polygon mirror <b>7</b><i>b </i>are not guided to the photoconductor <b>11</b><i>b </i>when the light beams deflected by the upper polygon mirror <b>7</b><i>a </i>optically scan the photoconductor <b>11</b><i>a</i>, and the light beams deflected by the upper polygon mirror <b>7</b><i>a </i>are not guided to the photoconductor <b>11</b><i>a </i>when the light beams deflected by the lower polygon mirror <b>7</b><i>b </i>optically scan the photoconductor <b>11</b><i>b. </i>

0046Namely, the optical scanning on the photoconductors <b>11</b><i>a </i>and <b>11</b><i>b </i>are alternately performed with a time lag. <figref idref="DRAWINGS">FIGS. 3A and 3B</figref> are views for explaining this state. For simplicity of the explanation, a light beam (in practice, four light beams) entering the polygon mirror type optical deflector <b>7</b> is shown as an incident light and light beams to be deflected are shown as a deflected light “a” and a deflected light “b”.

0047<figref idref="DRAWINGS">FIG. 3A</figref> depicts the state that the incident light enters the polygon mirror type optical deflector <b>7</b> and the deflected light “a” reflected and deflected by the upper polygon mirror <b>7</b><i>a </i>is guided to the optical scanning position. At this point, the deflected light “b” deflected by the lower polygon mirror <b>7</b><i>b </i>is not guided to the optical scanning position. <figref idref="DRAWINGS">FIG. 3B</figref> depicts the state that the deflected light “b” reflected and deflected by the lower polygon mirror <b>7</b><i>b </i>is guided to the optical scanning position. At this point, the deflected light “a” deflected by the upper polygon mirror <b>7</b><i>a </i>is not guided to the optical scanning position.

0048As shown in <figref idref="DRAWINGS">FIG. 3</figref>, it is preferable to use a proper light shielding unit SD to block the deflected light that is not guided to the optical scanning position in order that the deflected light by one of the polygon mirrors does not act as a ghost light while the deflected light by the other polygon mirror is guided to the optical scanning position. In practice, this can be easily achieved by having an inner wall of the sound-proof housing to be irreflexive.

0049As described above, in the embodiment shown in <figref idref="DRAWINGS">FIG. 1</figref>, the optical scanning (of a multi-beam system) of the photoconductors <b>11</b><i>a </i>and <b>11</b><i>b </i>are alternately performed. Therefore, for example, an electrostatic latent image of a black image can be written in the photoconductor <b>11</b><i>a </i>when light intensity of the light source is modulated by image signals of the black image in performing the optical scanning on the photoconductor <b>11</b><i>a</i>, and an electrostatic latent image of a magenta image can be written in the photoconductor <b>11</b><i>b </i>when the light intensity of the light source is modulated by the image signals of the magenta image in performing the optical scanning on the photoconductor <b>11</b><i>b. </i>

0050<figref idref="DRAWINGS">FIG. 4</figref> is a timing chart in the case of full-lighting in an effective scanning area when the black image and the magenta image are written with the common light source (semiconductor lasers <b>1</b> and <b>1</b>′ of <figref idref="DRAWINGS">FIG. 1</figref>). The solid line in <figref idref="DRAWINGS">FIG. 4</figref> indicates a portion corresponding to the write of the black image, and the broken line indicates a portion corresponding to the write of the magenta image. As described above, the timing of starting to write the black image and the magenta image is determined by detecting the light beam toward the optical scanning position with a synchronized light-reception unit (not shown in <figref idref="DRAWINGS">FIG. 1</figref>, usually the unit is a photodiode) arranged outside the effective scanning area.

0051In <figref idref="DRAWINGS">FIG. 4</figref>, light emission intensity of the light source in the writing time area of the black image is set to the same value as that of the magenta image. When there is a relative difference in transmittance or reflectance of the optical element in each optical path from the light source to the photoconductor <b>11</b><i>a </i>or <b>11</b><i>b</i>, and the photoconductors <b>11</b><i>a </i>and <b>11</b><i>b </i>are equal to each other in the light emission intensity of the light source, light volumes of the light beams reaching the photoconductors become different from each other. In this case, as shown in <figref idref="DRAWINGS">FIG. 5</figref>, the light volumes of the light beams reaching each photoconductor can be equalized by having the light emission intensity value of one of light sources differed from the value of the other light source when the light beams optically scan different photoconductors.

0052Instead of setting the light emission intensity in the light source to the different value for each photoconductor, for example, the light volumes of the light beams reaching photoconductors may be equalized by setting a ratio of transmittance to reflectance of the semitransparent mirror <b>4</b><i>a </i>of the half mirror prism <b>4</b> not to be 1 to 1, but by adjusting the ratio of transmittance to reflectance. In order to equalize the light volumes reaching different photoconductors, making light intensity values in the light sources different and adjusting the ratio of the transmittance to the reflectance of the semitransparent mirror <b>4</b><i>a </i>of the half mirror prism <b>4</b> may be combined.

0053The embodiment with reference to <figref idref="DRAWINGS">FIGS. 1 to 4</figref> is an optical scanner in which an optical scanning paths from one light source toward n (n=2) optical scanning positions are formed, the optical scanning paths toward the n optical scanning positions are sequentially selected with a time lag, and by an intensity modulation of light beams from one light source with image signals corresponding to the optical scanning position optically scanned by the selected optical scanning path, thereby the n optical scanning positions are optically scanned with the light beams from one light source, the optical scanner comprising m (=1) light source that emits p (=2) light beams, a light beam splitting units <b>2</b>, <b>2</b>′, <b>3</b>, <b>3</b>′, <b>4</b>, <b>5</b><i>a</i>, and <b>5</b><i>b </i>that form each light beam in a predetermined beam form while splitting each light beam from each light source into q (=2) light beams in a sub-scanning direction, a polygon mirror type optical deflector <b>7</b> that has a plurality of deflection reflection planes around a rotation axis and in which the number of light beams equal to (q·m·p=4) from the m light sources are incident to be deflected, n sets of scanning imaging optical systems <b>8</b><i>a</i>, <b>8</b><i>b</i>, <b>9</b><i>a</i>, <b>9</b><i>b</i>, <b>10</b><i>a</i>, and <b>10</b><i>b </i>that guide the n (=2) sets of light beams deflected by the polygon mirror type optical deflector to the corresponding optical scanning positions to form light spots, and an optical path selecting unit that selects the optical scanning path in each set for the n sets of light beams, wherein the polygon mirror type optical deflector <b>7</b> has the function of the optical path selecting unit.

0054In the polygon mirror type optical deflector <b>7</b>, the polygon mirrors <b>7</b><i>a </i>and <b>7</b><i>b </i>having N (=4) deflection reflection planes are integrally formed in q (=2) stages in the direction of the rotation axis, and the deflection reflection planes of the polygon mirrors in each stage are shifted from each other by the predetermined angle θ (=45°) in the rotation direction. The polygon mirror type optical deflector <b>7</b> also has the function of the optical path selecting unit.

0055Each of the light beam splitting unit splits the light beam into q (=2) light beams with the semitransparent mirror <b>4</b><i>a </i>and the reflection plane <b>4</b><i>b </i>and has an aperture <b>12</b> that restricts the light beam width between the light sources <b>1</b> and <b>1</b>′ and the semitransparent mirror <b>4</b><i>a </i>corresponding to the light source. By providing the aperture between the light source and the semitransparent mirror, the aperture <b>12</b> can be commonly used by the four light beams, the number of components can be reduced, and the difference in light spot quality between the light beams that optically scan different photoconductors can be reduced.

0056In the example described in <figref idref="DRAWINGS">FIG. 5</figref>, the light intensity of each light beam is set according to the optical scanning position where the optical scanning is performed. In the embodiment, m=1, n=p=q=2, and the multi-beam scanning is performed on each of the two optical scanning positions (photoconductors <b>11</b><i>a </i>and <b>11</b><i>b</i>) with two light beams.

0057In this embodiment, only the semiconductor laser <b>1</b> can be used as the light source. In this case, m=p=1, n=q=2, and each of the two optical scanning positions (photoconductors <b>11</b><i>a </i>and <b>11</b><i>b</i>) are optically scanned with one light beam.

0058While the light beam splitting unit splits each light beam into two light beams with the semitransparent mirror <b>4</b><i>a </i>and the reflection plane <b>4</b><i>b </i>in the embodiment, other method may also be used for splitting the light beam.

0059In an example shown in <figref idref="DRAWINGS">FIG. 6</figref>, a light beam is emitted from the light source (semiconductor laser <b>1</b> is exemplified). The light beam is then coupled in the parallel luminous flux state by the coupling lens <b>3</b>, and is split into q (=2) in the sub-scanning direction by passing through an aperture <b>12</b>A having q openings in the sub-scanning direction. When this method, the light beam can be separated without using the half mirror prism <b>4</b> and the aperture <b>12</b>A can be also used for the beam profiling, thereby reducing the number of components. As a result, cost reduction and downsizing of the apparatus can be achieved.

0060<figref idref="DRAWINGS">FIG. 7</figref> is a modification of <figref idref="DRAWINGS">FIG. 6</figref>. In <figref idref="DRAWINGS">FIG. 7</figref>, the light beam that is coupled in the parallel luminous flux state by the coupling lens <b>3</b> is split into q (=2) in the sub-scanning direction by passing through an aperture <b>12</b>B having q openings in the sub-scanning direction, and the light beam that has passed through one of the openings is shifted toward the sub-scanning (vertical direction of the drawing) by a prism <b>13</b>. The two beams can be easily separated by a necessary distance by shifting one of light beams toward the sub-scanning direction with the prism <b>13</b>.

0061In this example, since the distance between the two openings in an aperture <b>12</b>B can be made narrower as compared with the aperture <b>12</b>A of <figref idref="DRAWINGS">FIG. 6</figref>, the portion where the light intensity is large around the center of the light beam that has passed through the coupling lens <b>3</b> can be split into two light beams. Therefore, when compared with the example of <figref idref="DRAWINGS">FIG. 6</figref>, the light volume of the light beam used for the optical scanning can be more easily secured and a diameter of the spot can be further reduced.

0062<figref idref="DRAWINGS">FIGS. 8A</figref>, <b>8</b>B, and <b>8</b>C depict only the characteristic portion of a modification of the embodiment shown in <figref idref="DRAWINGS">FIG. 1</figref>. In the embodiment shown in <figref idref="DRAWINGS">FIG. 1</figref>, the polygon mirror type optical deflector <b>7</b> is one in which the upper polygon mirror <b>7</b><i>a </i>and the lower polygon mirror <b>7</b><i>b </i>are integrally formed in two stages in the direction of the rotation axis, and the deflection reflection plane of the upper polygon mirror <b>7</b><i>a </i>and the lower polygon mirror <b>7</b><i>b </i>are shifted in the rotation direction.

0063In the modification in which the characteristic portion is shown in <figref idref="DRAWINGS">FIGS. 8A and 8B</figref>, a polygon mirror type optical deflector <b>7</b>A has four deflection planes, and a size in the direction of the rotation axis of the deflection reflection plane is the degree to which the light beam separated in the sub-scanning direction can be entered. For simplicity of the explanation, it is assumed that the light source emits a single light beam that is split into two light beams in the sub-scanning direction by a light beam splitting unit (not shown), and it is also assumed that each split light beam is imaged as a long line image in the main scanning direction near the deflection reflection plane of the polygon mirror type optical deflector <b>7</b>A by an individual cylindrical lens (not shown).

0064Similarly to <figref idref="DRAWINGS">FIG. 2</figref>, the two light beams obtained by the split of the light beam with the light beam splitting unit are represented by reference signs L<b>11</b> and L<b>12</b>. The split of the light beam may be performed by the method shown in <figref idref="DRAWINGS">FIG. 2</figref>, or by the method shown in <figref idref="DRAWINGS">FIG. 6</figref> or <figref idref="DRAWINGS">FIG. 7</figref>.

0065As shown in <figref idref="DRAWINGS">FIGS. 8A and 8B</figref>, the light beams L<b>11</b> and L<b>12</b> are parallel to each other in the sub-scanning direction. However, as shown in <figref idref="DRAWINGS">FIG. 8A</figref>, the optical path of the light beam L<b>12</b> is curved by mirrors M<b>1</b> and M<b>2</b> when viewed from the direction of the rotation axis of the polygon mirror type optical deflector <b>7</b>A. In the state that the light beams L<b>11</b> and L<b>12</b> enter the deflection reflection plane of the polygon mirror type optical deflector <b>7</b>A, when the luminous fluxes are projected to the plane orthogonal to the rotation axis, as shown in FIG. <b>8</b>A, there is a difference between the light beams by an angle ξ. For example, when the angle ξ is set to 45°, the deflections of the light beams L<b>11</b> and L<b>12</b> by the polygon mirror type optical deflector <b>7</b>A become equal to the deflections of the light beams L<b>11</b> and L<b>12</b> by the polygon mirror type optical deflector <b>7</b> shown in <figref idref="DRAWINGS">FIG. 1</figref>.

0066However, in the case of <figref idref="DRAWINGS">FIGS. 8A and 8B</figref>, unlike the case of <figref idref="DRAWINGS">FIG. 1</figref>, since the light beams L<b>11</b> and L<b>12</b> are different from each other in the optical path from the light source to the polygon mirror type optical deflector <b>7</b>A, in the cylindrical lens that images the light beam as the long line image in the main scanning direction near a deflection reflection plane of a polygon mirror type optical deflector <b>7</b>, it is necessary that a focal distance and/or the arrangement of the cylindrical lens for the light beam L<b>11</b> and for the light beam L<b>12</b> to be made different from each other.

0067Namely, in the embodiment in which its characteristic portion is shown in <figref idref="DRAWINGS">FIGS. 8A and 8B</figref>, the polygon mirror type optical deflector <b>7</b>A is the polygon mirror having N (=4) deflection reflection planes. The directions of the n (=2) sets of light beams L<b>11</b> and L<b>12</b> entering the deflection reflection plane of the polygon mirror type optical deflector <b>7</b>A from the light source side are set so as to mutually form the angle ξ when viewed from the direction of the rotation axis of the polygon mirror type optical deflector <b>7</b>A. The optical path selecting unit is formed by the setting of the polygon mirror type optical deflector <b>7</b>A and the incident directions of the n sets of light beams L<b>11</b> and L<b>12</b> (specifically, an arrangement of the mirrors M<b>1</b> and M<b>2</b>).

0068<figref idref="DRAWINGS">FIG. 8C</figref> is a modification of <figref idref="DRAWINGS">FIGS. 8A and 8B</figref>. In the projection of the light beams L<b>11</b> and L<b>12</b> to the plane orthogonal to the rotation axis of a polygon mirror type optical deflector <b>7</b>B (having four deflection reflection planes), similarly to <figref idref="DRAWINGS">FIG. 8A</figref>, the light beams L<b>11</b> and L<b>12</b> have the angle ξ, and the light beams L<b>11</b> and L<b>12</b> form the angle η as shown in <figref idref="DRAWINGS">FIG. 8</figref> when viewed from the direction orthogonal to the rotation axis. When the angle η is given between the light beams L<b>11</b> and L<b>12</b> when viewed from the direction orthogonal to the rotation axis, the size in the direction of the rotation axis of the polygon mirror type optical deflector <b>7</b>B can be reduced when compared with the polygon mirror type optical deflector <b>7</b>A.

0069Although the number N of deflection reflection planes of the polygon mirror type optical deflector is set to 4 in the above descriptions, needless to say, it is also possible to use the polygon mirror type optical deflector having the deflection reflection planes of N=2, N=3, or equal to or more than 5. The polygon mirror type optical deflector, in which the polygon mirrors having equal to or more than five deflection reflection planes are integrally stacked in two stages in a direction of the rotation axis and the deflection reflection planes of the polygon mirrors are shifted from each other in a rotation direction, is described in Japanese Patent Application Laid-Open No. 2001-83452.

0070With reference to <figref idref="DRAWINGS">FIGS. 9 and 10</figref>, an embodiment with respect to an image forming apparatus to which the optical scanner in the above embodiment is applied will be explained.

0071<figref idref="DRAWINGS">FIG. 9</figref> depicts an optical portion of the optical scanner when viewed from the sub-scanning direction, i.e. the direction of the rotation axis of the polygon mirror type optical deflector <b>7</b>. For simplicity of the explanation, the mirrors for folding the optical path from the polygon mirror type optical deflector <b>7</b> to the optical scanning position are omitted, and the optical path is drawn to be straight.

0072In the optical scanner shown in <figref idref="DRAWINGS">FIG. 9</figref>, m=q=2, p=1, and n=4, and the four optical scanning positions are optically scanned with one light beam. The photoconductors <b>11</b>Y, <b>11</b>M, <b>11</b>C, and <b>11</b>K are arranged in each optical scanning position, and the electrostatic latent images formed on the four photoconductors are individually visualized by magenta toner, yellow toner, cyan toner, and black toner to form the color image.

0073In <figref idref="DRAWINGS">FIG. 9</figref>, reference signs <b>1</b>YM and <b>1</b>CK represent the semiconductor laser. The semiconductor lasers <b>1</b>YM and <b>1</b>CK emit one light beams. The semiconductor laser <b>1</b>YM is alternately intensity-modulated by the image signal corresponding to a yellow image and the image signal corresponding to a magenta image. The semiconductor laser <b>1</b>CK is alternately intensity-modulated by the image signal corresponding to a cyan image and the image signal corresponding to a black image.

0074The light beam emitted from the semiconductor laser <b>1</b>YM is formed in the parallel luminous flux by a coupling lens <b>3</b>YM and beam-shaped by passing through an aperture <b>12</b>YM, the light beam to enter a half mirror prism <b>4</b>YM and split into two light beams in the sub-scanning direction. The half mirror prism <b>4</b>YM is similar to the half mirror prism <b>4</b> explained with reference to <figref idref="DRAWINGS">FIG. 2</figref>. One of the two split light beams is used for the write of the yellow image, and the other light beam is used for the write of the magenta image.

0075The two light beams split in the sub-scanning direction are focused in the sub-scanning direction by cylindrical lenses <b>5</b>Y and <b>5</b>M (arranged to overlap each other in the sub-scanning direction) to enter the polygon mirror type optical deflector <b>7</b>. The polygon mirror type optical deflector <b>7</b> is similar to the polygon mirror type optical deflector explained with reference to <figref idref="DRAWINGS">FIGS. 1 and 3</figref>, the polygon mirrors having the four deflection reflection planes are integrally formed in two stages in the direction of the rotation axis, and the deflection reflection planes of the polygon mirrors are shifted from each other in the rotation direction. The long line image in the main scanning direction by the cylindrical lenses <b>5</b>Y and <b>5</b>M is imaged near the deflection reflection plane position of each polygon mirror.

0076The light beams deflected by the polygon mirror type optical deflector <b>7</b> are transmitted through first scanning lenses <b>8</b>Y and <b>8</b>M and second scanning lenses <b>10</b>Y and <b>10</b>M respectively, and the light beams form the light spots on optical scanning positions <b>11</b>Y and <b>11</b>M by the operation of the lenses and optically scan the optical scanning position.

0077Similarly, the light beam emitted from the semiconductor laser <b>1</b> CK is formed in the parallel luminous flux by a coupling lens <b>3</b>CK and beam-shaped by passing through an aperture <b>12</b>CK. The light beam then enters a half mirror prism <b>4</b>CK and split into two light beams in the sub-scanning direction. The half mirror prism <b>4</b>CK is similar to the half mirror prism <b>4</b>YM. One of the two split light beams is used for the write of the cyan image, and the other light beam is used for the write of the black image.

0078The two light beams split in the sub-scanning direction are focused in the sub-scanning direction by cylindrical lenses <b>5</b>C and <b>5</b>K (arranged to overlap each other in the sub-scanning direction) to be incident to the polygon mirror type optical deflector <b>7</b>. The light beams deflected by the polygon mirror type optical deflector <b>7</b> are transmitted through first scanning lenses <b>8</b>C and <b>8</b>K and second scanning lenses <b>10</b>C and <b>10</b>K, respectively, and the light beams form the light spots on optical scanning positions <b>11</b>C and <b>11</b>K by the operation of the lenses and optically scan the optical scanning position.

0079The portion represented by a reference sign <b>20</b> in <figref idref="DRAWINGS">FIG. 10</figref> is the optical scanner, and it is the unit described in of <figref idref="DRAWINGS">FIG. 9</figref>. As shown in <figref idref="DRAWINGS">FIG. 10</figref>, one of the light beams deflected by the upper polygon mirror of polygon mirror type optical deflector <b>7</b> is guided to the optical scanning position of the photoconductor <b>11</b>M by the optical path curved with optical path folding mirrors mM<b>1</b>, mM<b>2</b>, and mM<b>3</b>, and the other light beam is guided to the optical scanning position of the photoconductor <b>11</b>C by the optical path curved with optical path folding mirrors mC<b>1</b>, mC<b>2</b>, and mC<b>3</b>.

0080One of the light beams deflected by the lower polygon mirror of polygon mirror type optical deflector <b>7</b> is guided to the optical scanning position of the photoconductor <b>11</b>Y by the optical path curved with an optical path folding mirrors mY, and the other light beam is guided to the optical scanning position of the photoconductor <b>11</b>K by the optical path curved with an optical path folding mirrors mK.

0081Accordingly, the light beams from the m (=2) semiconductor lasers <b>1</b>YM and <b>1</b>CK are split into four light beams by half mirror prisms <b>4</b>YM and <b>4</b>CK, and the four photoconductors <b>11</b>Y, <b>11</b>M, <b>11</b>C, and <b>11</b>K are optically scanned with the four light beams. The photoconductors <b>11</b>Y and <b>11</b>M are alternately scanned optically with the light beams into which the light beam from the semiconductor laser <b>1</b>YM is split while the polygon mirror type optical deflector <b>7</b> is rotated. The photoconductors <b>11</b>C and <b>11</b>K are alternately scanned optically with the light beams into which the light beam from the semiconductor laser <b>1</b>CK is split while the polygon mirror type optical deflector <b>7</b> is rotated.

0082Each of the photoconductors <b>11</b>Y, <b>11</b>M, <b>11</b>C, and <b>11</b>K is uniformly rotated clockwise to be uniformly charged by charging rollers TY, TM, TC, and TK that are of a charging unit. Each of the yellow, the magenta, the cyan, and the black color images is written in the photoconductors <b>11</b>Y, <b>11</b>M, <b>11</b>C, and <b>11</b>K by the optical scanning on the corresponding light beam to form the corresponding electrostatic latent image (negative latent image).

0083The electrostatic latent images are reversely developed by development devices GY, GM, GC, and GK to form the yellow, magenta, cyan, and black toner images on the photoconductors <b>11</b>Y, <b>11</b>M, <b>11</b>C, and <b>11</b>K.

0084The toner images are transferred to a transfer sheet (not shown). The transfer sheet is conveyed by a conveying belt <b>17</b>. The yellow toner image is transferred from the photoconductor <b>11</b>Y to the transfer sheet by a transfer device <b>15</b>Y, and the magenta, the cyan, and the black toner images are sequentially transferred from the photoconductors <b>11</b>M, <b>11</b>C, and <b>11</b>K to the transfer sheet by transfer devices <b>15</b>M, <b>15</b>C, and <b>15</b>K, respectively.

0085Thus, the yellow, the magenta, the cyan, and the black toner images overlap each other on the transfer sheet to synthetically form a color image. The color image is fixed onto the transfer sheet by a fixing device <b>19</b> to obtain color images.

0086The embodiment shown in <figref idref="DRAWINGS">FIGS. 9 and 10</figref> is a tandem type image forming apparatus in which the electrostatic latent images are individually formed by the optical scanning on the plurality of photoconductors, the electrostatic latent images are visualized to form toner images, and the toner images are transferred to a same sheet-like recording medium to synthetically perform image formation, wherein the number of photoconductors is four, two light sources <b>1</b>YM and <b>1</b>CK are used, the optical scanner is formed so that each two photoconductors are optically scanned with light beams from each light source, and the electrostatic latent images formed on the four photoconductors <b>11</b>M, <b>11</b>Y, <b>11</b>C, and <b>11</b>K are individually visualized with the magenta toner, yellow toner, cyan toner, and black toner to form a color image.

0087While in the embodiment of the image forming apparatus, the optical scanning on each photoconductor is performed by a single beam method, it is also possible that each light source side is formed as shown in <figref idref="DRAWINGS">FIG. 1</figref>, and the optical scanning on the photoconductor is performed by a multi-beam method.

0088As described above, according to the optical scanner and the image forming apparatus of the embodiments, the polygon mirror type optical deflector constitutes at least a part of the optical path selecting unit, and the polygon mirror type optical deflector has a plurality of deflection reflection planes. Accordingly, at least four optical scanning can be performed per one rotation of the polygon mirror type optical deflector, and the optical scanning speed can be increased for n optical scanning positions although the number m of light sources is smaller than the number n of optical scanning positions.

0089Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.